Method of forming a sputtering target

ABSTRACT

A sputtering target, including a sputtering layer and a support structure. The sputtering layer includes an alkali-containing transition metal. The support structure includes a second material that does not negatively impact the performance of a copper indium selenide (CIS) based semiconductor absorber layer of a solar cell. The sputtering layer directly contacts the second material.

FIELD OF THE INVENTION

The present invention relates generally to the field of sputteringtargets used in the manufacturing of photovoltaic devices, and morespecifically to forming thin-film solar cells by sputter depositing analkali-containing transition metal electrode.

BACKGROUND OF THE INVENTION

Copper indium diselenide (CuInSe₂, or CIS) and its higher band gapvariants copper indium gallium diselenide (Cu(In,Ga)Se₂, or CIGS),copper indium aluminum diselenide (Cu(In,Al)Se₂), copper indium galliumaluminum diselenide (Cu(In,Ga,Al)Se₂) and any of these compounds withsulfur replacing some of the selenium represent a group of materials,referred to as copper indium selenide CIS based alloys, have desirableproperties for use as the absorber layer in thin-film solar cells. Tofunction as a solar absorber layer, these materials should be p-typesemiconductors.

SUMMARY OF THE INVENTION

One embodiment of this invention provides a sputtering target includinga sputtering layer comprising a first material which comprises analkali-containing transition metal and a support structure comprising asecond material, wherein the sputtering layer directly contacts thesecond material and the second material does not negatively impact theperformance of a copper indium selenide (CIS) based semiconductorabsorber layer of a solar cell.

Another embodiment of the invention provides a method of manufacturing asputtering target including forming a sputtering layer comprising afirst material which comprises an alkali-containing transition metaldirectly on a second material of a support structure, wherein the secondmaterial does not negatively impact the performance of a copper indiumselenide (CIS) based semiconductor absorber layer of a solar cell.

Another embodiment of the invention provides a method of making a solarcell comprising a substrate, a first electrode, at least one p-typesemiconductor absorber layer comprising a copper indium selenide (CIS)based alloy material, an n-type semiconductor layer and a secondelectrode, the method including sputtering the first electrodecomprising an alkali-containing transition metal layer from a target.The target may comprise a sputtering layer including a first materialwhich comprises an alkali-containing transition metal sputtering layerand a support structure comprising a second material, wherein thesputtering layer of the target directly contacts the second material.The second material is selected such that atoms which negatively impactthe performance of the CIS based alloy material are not incorporatedinto the p-type semiconductor absorber layer from the alkali containingtransition metal layer of the first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a tubular sputtering target of afirst embodiment of the invention.

FIG. 2 shows a cross-sectional view of a tubular sputtering target of asecond embodiment of the invention comprising a barrier layer.

FIG. 3 shows a cross-sectional view of an alternate tubular sputteringtarget of the second embodiment of the invention comprising two barrierlayers.

FIG. 4 shows a cross-sectional view of a tubular sputtering target ofthe second embodiment of the invention comprising three barrier layers.

FIG. 5 shows a cross-sectional view of a planar sputtering target of athird embodiment the invention.

FIG. 6 shows a cross-sectional view of a planar sputtering target of thethird embodiment of the invention comprising a barrier layer.

FIG. 7 shows a cross-sectional view of an alternate planar sputteringtarget of the third embodiment of the invention comprising two barrierlayers.

FIG. 8 a shows a Secondary Ion Mass Spectrometry (SIMS) spectra resultof a comparative low temperature thin film example. FIG. 8 b shows aSIMS spectra result of a first exemplary low temperature thin filmexample. FIG. 8 c shows a SIMS spectra result of a second exemplary lowtemperature thin film example.

FIG. 9 is a schematic side cross-sectional view of a CIS based solarcell manufactured according to a method of the present invention.

FIG. 10 shows a highly simplified schematic diagram of a top view of amodular sputtering apparatus that can be used to manufacture the solarcell depicted in FIG. 9 according to a method of an embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

During production of thin-film solar cells, electrodes may be depositedonto a substrate by sputtering sodium-containing molybdenum (e.g.,molybdenum doped with sodium). In some instances, the sputtering targetcomprises a support, for example a backing layer comprised of stainlesssteel, and the sodium-containing molybdenum sputtering layer formed overthe support.

A sodium and molybdenum premixed powder, such as a powder that comprisesa mixture of Mo and a sodium salt, such as Na₂MoO₄, is consolidated as asputtering layer by a high temperature process, at between 500-1500° C.onto a stainless steel support, such as a stainless steel backing tube.Stainless steel comprises iron (Fe) as well as chromium (Cr) and nickel(Ni) alloying elements which are susceptible to migrating into thesodium-containing molybdenum sputtering layer. Once these impuritieshave diffused into the sodium-containing molybdenum sputtering layer,they may also be sputtered together with the sodium-containingmolybdenum during deposition of the electrode of a thin-film solar celland become incorporated into the electrode. Thereafter, these impuritiesmay migrate into a CIGS absorber layer formed over the electrode, thussignificantly decreasing conversion efficiency of the solar cell.

Iron incorporation shifts the electrical/optical properties of a CIGSfilm, thus resulting in a deteriorated spectral response of a solar cell(see Sakurai et. al., Mater. Res. Soc. Symp. Proc. Vol. 865, F14.12.1)).Similarly, it is believed that diffusion of Ni and Cr atoms may alsonegatively impact properties of CIS absorber layers. Thus, Fe, Ni and Crare three elements which negatively impact the performance (such asdevice efficiency) of a CIS absorber layer.

As used herein, the phrase “does not negatively impact the performanceof a CIS absorber layer”, or the like, includes the following twodefinitions. The first definition includes the situation where thediffusion of certain materials into the CIS absorber layer of a solarcell does not impact the absorber layer performance (such as deviceefficiency). For example, when material originating from a target (suchas Mo barrier material) is incorporated into an electrode duringsputtering, and subsequently diffuses into the CIS absorber layer of asolar cell, the material does not impact the CIS absorber layerperformance in such a way as to reduce device efficiency. The seconddefinition includes the situation where atoms from a target which donegatively impact the performance of a CIS absorber layer do not diffuseor are not incorporated into the CIS absorber layer of a solar cell atall. For example, for a Cr barrier layer under a Mo barrier layer, theCr does not subsequently diffuse or is not subsequently incorporatedinto the CIS based absorber layer at all.

To solve the problem of Fe, Ni and/or Cr incorporation in a CIS basedabsorber layer of a solar cell, one embodiment of the present inventionprovides a method of making a solar cell formed by depositing a firstelectrode comprising an alkali-containing transition metal layer over asubstrate, depositing at least one p-type semiconductor absorber layercomprising a copper indium selenide (CIS) based alloy material over thefirst electrode, depositing an n-type semiconductor layer over thep-type semiconductor layer, and depositing a second electrode over then-type semiconductor layer. If desired, the layers may be deposited inreverse order if a transparent substrate is used. The step of depositingthe first electrode comprises sputtering the an alkali-containingtransition metal layer from a target by DC sputtering, AC sputtering, orRF sputtering a sputtering layer of a sputtering target.

The target may comprise a sputtering layer comprising a first material,and a support structure comprising a second material. The sputteringlayer directly contacts the second material. The second material isselected such that atoms which negatively impact the performance of theCIS based alloy material, such as Fe, Ni and/or Cr, are not incorporatedinto the p-type semiconductor absorber layer from the alkali containingtransition metal layer of the first electrode.

The sputtering layer may comprise a first material comprising analkali-containing transition metal. The transition metal of thealkali-containing transition metal sputtering layer may be one of Mo, W,Ta, V, Ti, Nb, Zr, or alloys thereof. The alkali metal of thealkali-containing transition metal sputtering layer may be one of Li,Na, K, or combinations thereof. In one embodiment, the alkali-containingtransition metal layer comprises molybdenum intentionally doped with atleast one alkali element, such as sodium (herein referred to assodium-containing molybdenum). An example of a sodium-containingmolybdenum sputtering layer comprises Na₂MoO₄ combined with Mo. Also, inembodiments of the present invention, the first material of thesputtering layer may comprise 0.5 to 10 wt % sodium, 0-50 wt % oxygenand the balance molybdenum. The present inventors have found that Fe,Ni, and Cr diffuse more readily into sodium-containing molybdenum thaninto Mo alone. Thus, while contact between stainless steel and a Mosputtering layer is acceptable, contact between stainless steel and asodium-containing molybdenum sputtering layer leads to contamination ofthe sputtering layer with one or more of Fe, Ni and Cr.

In a first embodiment of the invention, the second material of thetarget support structure comprises a material other than stainlesssteel, such as Mo. Generally, however, the second material of the targetsupport structure may be selected from a group consisting of Mo, W, Ta,V, Ti, Nb, Zr, alloys thereof and nitrides thereof, rather thanstainless steel.

In a second embodiment of the present invention, the support structureof the target may comprise at least one barrier layer comprising thesecond material formed on a backing structure of a third material, suchas stainless steel, that is different from the second material. Thebarrier layer is capable of blocking diffusion of atoms, for example Fe,Ni and/or Cr from the backing structure into the sputtering layer.

For example, the backing structure may comprise stainless steel whilethe barrier layer is capable of blocking diffusion of at least one ofFe, Ni or Cr atoms (which could otherwise negatively impact theperformance of the CIS based semiconductor absorber layer) from thestainless steel toward the sputtering layer of the target. The barrierlayer may comprise one or more layers of Mo, W, Ta, V, Ti, Nb, Zr,alloys thereof and nitrides thereof. For example, the barrier layer maycomprise a first barrier layer comprising Cr formed directly on and incontact with a backing structure comprising stainless steel, and asecond barrier layer comprising Mo. The second barrier layer is formedbetween the first barrier layer and a sputtering layer comprisingsodium-containing molybdenum. In some embodiments, an additional thirdbarrier layer comprising Nb is formed between the second barrier layerand the sputtering layer.

In some sputtering systems, tubular rotatable targets are used while inothers, planar or stacked targets are used. For example, U.S. Pat. No.4,356,073, which is hereby incorporated by reference herein in itsentirety, discloses a rotatable tubular target. Also, U.S. Pat. No.5,904,966, which is incorporated by reference herein in its entirety,provides a multi-layer target as a rotatable or static tubularstructure, or alternatively, in the form of a flat plate. Thus,embodiments of the present invention provide either a rotatable orstatic tubular sputtering target, such as comprising a backing structureas a hollow support tube, or a planar sputtering target comprising abacking structure as a planar support plate.

One advantage of the second embodiment of the invention is a sputteringtarget having a barrier layer capable of blocking diffusion of materialsfrom a backing structure that could otherwise negatively impact theperformance of the CIS based semiconductor absorber layer of a solarcell. For example, by providing a sputtering layer over a barrier layercapable of blocking at least one of Fe, Ni or Cr atoms that canotherwise diffuse from a stainless steel backing structure toward thesputtering layer, the concentration of these atoms in the sputteringlayer of the target is thereby minimized. For example, the term capableof blocking means that the concentration of Fe, Ni or Cr atoms in thesputtering layer is decreased by at least an order of magnitude relativeto a sputtering target with no such barrier layer. Thus, when thesputtering layer of the target is sputtered to form the first electrode,the concentration of Fe, Ni or Cr in the resulting first electrode isalso minimized. Therefore the diffusion of Fe, Ni or Cr to the CIS basedsemiconductor absorber layer of the solar cell deposited over the firstelectrode is also minimized.

For example, as illustrated in FIG. 1, a sputtering target 1 a of afirst embodiment of the invention includes a sputtering layer 10comprising a first material which comprises an alkali-containingtransition metal and a support structure 13 comprising a secondmaterial. The sputtering layer 10 directly contacts the second materialand the second material does not negatively impact the performance of acopper indium selenide (CIS) based semiconductor absorber layer of asolar cell. In the embodiment illustrated in FIG. 1, the supportstructure 13 comprises a backing structure tube comprising the secondmaterial. In other words, the backing structure tube comprises Mo, W,Ta, V, Ti, Nb, Zr, alloys thereof or nitrides thereof. Asodium-containing molybdenum sputtering layer 10 is formed directly onsuch backing structure.

The support structure of a second embodiment of the invention comprisesone or more barrier layers. For example, as illustrated in FIG. 2, asputtering target 1 b of a second embodiment of the invention comprisesa multi-layer tubular structure comprising sputtering layer 10 and asupport structure 13. Sputtering layer 10 may comprise a first materialwhich comprises an alkali-containing transition metal, such assodium-containing molybdenum, and the support structure 13 may comprisea second material barrier layer 12 on a backing structure 14. In thisembodiment, the sputtering layer 10 directly contacts the secondmaterial 12 formed on the backing structure 14 comprising stainlesssteel or a similar Fe, Ni or Cr material. Barrier layer 12 may be a foilor a deposited layer.

FIG. 3 illustrates an alternative embodiment of a sputtering target lcof the present invention having a similar tubular structure as that ofFIG. 2 except that barrier layer comprises a first barrier layer 12 aand a second barrier layer 12 b. The first barrier layer 12 a maycomprise Cr and the second barrier layer 12 b may comprise Mo.Additionally, as illustrated in FIG. 4, a sputtering target 1 d may havea similar structure as the sputtering target 1 c with the addition of athird barrier layer 12 c comprising Nb formed between the second barrierlayer 12 b and the sputtering layer 10.

Rather than providing a tubular target, a third embodiment of asputtering target of the present invention is provided as a planarsputtering target. For example, FIG. 5 illustrates a planar sputteringtarget 2 a comprising a similar multilayer configuration as thesputtering target illustrated in FIG. 1 except that all layers areplanar shaped rather than having an annular shape. Alternatively, asillustrated in FIG. 6, planar sputtering target 2 b comprises a barrierlayer 12 formed over a backing structure 14 and a sputtering layer 10formed over the barrier layer 12 as a planar version of the multilayerconfiguration of the sputtering target of FIG. 2. Additionally, a planarsputtering target 2 c illustrated in FIG. 7 comprises multiple barrierlayers. For example a first barrier layer 12 a is formed over backingstructure 14, and a second barrier layer 12 b is formed between firstbarrier layer 12 a and sputtering layer 10 as a planar version of themultilayer configuration of the tubular sputtering target of FIG. 3.

While the embodiments with respect to sputtering targets 1 a, 1 b, 1 c,1 d, 2 a, 2 b and 2 c have been described with up to three barrierlayers comprising specific barrier materials, for example a firstbarrier layer comprising Cr, a second barrier layer comprising Mo and athird barrier layer comprising Nb, the invention is not so limited. Infact, the present invention may generally comprise any number of barrierlayers (including no barrier layers), at least one of which may compriseat least one of Cr, Mo and Nb. Other barrier layers comprising materialsthat are capable of preventing the diffusion of atoms to the sputteringlayer that are deleterious to the absorber layer of a solar cell mayalso be used.

The barrier layer(s) 12 may be formed on the backing structure by anysuitable method. For example, the barrier layer may be formed as a filmof about at least 1 μm in thickness by depositing the second materialonto the backing structure by sputtering, pressing or thermal spraying(including coating processes selected from the group comprising plasmaspray, cold spray, high velocity oxygen fuel (“HVOF”), twin wire arcspray (“TWAS”) and flame spray). Alternatively, the barrier may beformed by providing a barrier foil having a thickness of about 1-1000μm, attaching the barrier foil to the backing structure to form thebarrier layer, and hot pressing the first material (e.g., a sodium andmolybdenum premixed powder, or sodium-containing molybdenum) to thebarrier layer to form the sputtering layer and to bond the barrier layerto the backing structure. The process of hot pressing the first materialto the barrier layer may include a number of conventional methods suchas vacuum hot pressing, hot isostatic pressing (“HIP”), uniaxialpressing, or the like.

Comparative Low-Temperature Thin Film Example:

An about 1.1-1.2 μm layer of sodium-containing molybdenum was depositedon a bare 430 SST stainless steel substrate and annealed at 400° C. forone hour thereby forming a thin-film sample. FIG. 8 a is a SIMS depthprofile of this comparative sputtering target showing thatsodium-containing molybdenum alone is not capable of blocking thediffusion of Fe atoms from the stainless steel backing layer.

First Exemplary Low-Temperature Thin Film Example:

An about 1.3 μm barrier layer of Mo was deposited on a bare 430 SSTstainless steel substrate. An about 1.1-1.2 μm layer ofsodium-containing molybdenum was deposited on the Mo layer and annealedat 400° C. for one hour thereby forming a first thin-film sample. FIG. 8b is a SIMS depth profile of this first sample showing that a Mo barrierlayer is capable of reducing the diffusion of Fe atoms from thestainless steel backing layer toward the sodium-containing molybdenumsputtering layer. It is noted that this first thin-film examplecomprises a configuration of layers similar to those of some embodimentsof sputtering targets of the present invention, and is only provided asan example which is not intended to limit the present invention.

Second Exemplary Low-Temperature Thin Film Example:

An about 0.6 μm first barrier layer of Cr was deposited on a bare 430SST stainless steel substrate. An about 0.7 μm second barrier layer ofMo was deposited over the first barrier layer. An about 1.1-1.2 μm layerof sodium-containing molybdenum was then deposited on the second barrierlayer. The deposited layers were then annealed at 400° C. for one hourthereby forming a second thin-film sample. FIG. 8 c is a SIMS depthprofile of this second sample showing that the Cr and Mo barrier layersare capable of reducing the diffusion of Fe atoms from the stainlesssteel backing layer toward the sodium-containing molybdenum layer. It isnoted that this second example comprises a configuration of layerssimilar to those of some embodiments of sputtering targets of thepresent invention, and is only provided as an example which is notintended to limit the present invention.

In summary, as shown in the SIMS depth profiles through a comparativesample of a sputtering target, as shown in FIG. 8 a, Fe substantiallydiffuses into sodium-containing molybdenum even at a temperature as lowas 400° C., which is a lower temperature is utilized in several hightemperature pressing processes (conventionally at 500-1500° C.) used tomanufacture targets. As shown in FIGS. 8 a and 8 b, a Mo barrier layerreduces Fe concentration in sodium-containing molybdenum by one to twoorders of magnitude. For example, the concentration of Fe is greaterthan about 1×10²⁰ atoms/cm³, such as about 10²⁰-10²¹ atoms/cm³ insodium-containing molybdenum, but 10¹⁹-10²⁰ in Mo. As shown in FIG. 8 c,a barrier formed of a first barrier layer comprising Cr and a secondbarrier layer comprising Mo also reduces Fe diffusion. The reductionshould be even more pronounced for sputtering layers formed at a highertemperature, such as by a vacuum hot pressing, uniaxial pressing or HIPprocess.

The sputtering target of the embodiments of the invention may be used inany suitable solar cell or other device which is made by a manufacturingprocess comprising a sputtering step. For example, as illustrated inFIG. 9, the method of making a solar cell comprises providing asubstrate 100, and depositing a first (lower) electrode 200 over thesubstrate.

Optionally, the first electrode 200 of the solar cell may comprise oneor more barrier layers 201 located under the alkali-containingtransition metal layer 202, and/or one or more adhesion layers 203located over the alkali-containing transition metal layer 202. In someembodiments, the barrier layer 201 is denser than the adhesion layer203, and substantially prevents alkali diffusion from thealkali-containing transition metal layer 202 into the substrate 100. Inthese embodiments, alkali may diffuse from the alkali-containingtransition metal layer 202, through the lower density adhesion layer203, into the at least one p-type semiconductor absorber layer 301during and/or after the step of depositing the at least one p-typesemiconductor absorber layer 301. The optional barrier layer 201 andadhesion layer 203 may comprise any suitable materials. For example,they may be independently selected from a group consisting Mo, W, Ta, V,Ti, Nb, Zr, Cr, TiN, ZrN, TaN, VN, V₂N or combinations thereof. In oneembodiment, while the barrier layer 201 may be oxygen free, thealkali-containing transition metal layer 202 and/or the adhesion layer203 may contain oxygen and/or be deposited at a higher pressure than thebarrier layer 201 to achieve a lower density than the barrier layer 201.

Alternatively, the optional one or more barrier layers 201 and/oroptional one or more adhesion layers 203 may be omitted. When theoptional one or more adhesion layers 203 are omitted, the at least onep-type semiconductor absorber layer 301 is deposited over thealkali-containing transition metal layer 202, and alkali may diffusefrom the alkali-containing transition metal layer 202 into the at leastone p-type semiconductor absorber layer 301 during or after thedeposition of the at least one p-type semiconductor absorber layer 301.

In preferred embodiments, the p-type semiconductor absorber layer 301may comprise a CIS based alloy material selected from copper indiumselenide, copper indium gallium selenide, copper indium aluminumselenide, or combinations thereof. Layer 301 may have a stoichiometriccomposition having a Group Ito Group III to Group VI atomic ratio ofabout 1:1:2, or a non-stoichiometric composition having an atomic ratioof other than about 1:1:2. Preferably, layer 301 is slightly copperdeficient and has a slightly less than one copper atom for each one ofGroup III atom and each two of Group VI atoms. The step of depositingthe at least one p-type semiconductor absorber layer may comprisereactively AC sputtering the semiconductor absorber layer from at leasttwo electrically conductive targets in a sputtering atmosphere thatcomprises argon gas and a selenium containing gas (e.g. selenium vaporor hydrogen selenide). For example, each of the at least twoelectrically conductive targets comprises copper, indium and gallium;and the CIS based alloy material comprises copper indium galliumdiselenide.

An n-type semiconductor layer 302 may then be deposited over the p-typesemiconductor absorber layer 301. The n-type semiconductor layer 302 maycomprise any suitable n-type semiconductor materials, for example, butnot limited to ZnS, ZnSe or CdS.

A second electrode 400, also referred to as a transparent top electrode,is further deposited over the n-type semiconductor layer 302. Thetransparent top electrode 400 may comprise multiple transparentconductive layers, for example, but not limited to, one or more of anIndium Tin Oxide (ITO), Zinc Oxide (ZnO) or Aluminum Zinc Oxide (AZO)layers 402 located over an optional resistive Aluminum Zinc Oxide (RAZO)layer 401. Of course, the transparent top electrode 400 may comprise anyother suitable materials, for example, doped ZnO or SnO.

Optionally, one or more antireflection (AR) films (not shown) may bedeposited over the transparent top electrode 400, to optimize the lightabsorption in the cell, and/or current collection grid lines may bedeposited over the top conducting oxide.

Alternatively, the solar cell may be formed in reverse order. In thisconfiguration, a transparent electrode is deposited over a substrate,followed by depositing an n-type semiconductor layer over thetransparent electrode, depositing at least one p-type semiconductorabsorber layer over the n-type semiconductor layer, and depositing a topelectrode comprising an alkali-containing transition metal layer overthe at least one p-type semiconductor absorber layer. The substrate maybe a transparent substrate (e.g., glass) or opaque (e.g., metal). If thesubstrate used is opaque, then the initial substrate may be delaminatedafter the steps of depositing the stack of the above described layers,and then bonding a glass or other transparent substrate to thetransparent electrode of the stack.

More preferably, the steps of depositing the first electrode 200,depositing the at least one p-type semiconductor absorber layer 301,depositing the n-type semiconductor layer 302, and depositing the secondelectrode 400 comprise sputtering the alkali-containing transition metallayer 202, the p-type absorber layer 301, the n-type semiconductor layer302 and one or more conductive films of the second electrode 400 overthe substrate 100 (preferably a web substrate in this embodiment) incorresponding process modules of a plurality of independently isolated,connected process modules without breaking vacuum, while passing the websubstrate 100 from an input module to an output module through theplurality of independently isolated, connected process modules such thatthe web substrate continuously extends from the input module to theoutput module while passing through the plurality of the independentlyisolated, connected process modules. Each of the process modules mayinclude one or more sputtering targets for sputtering material over theweb substrate 100.

As discussed above, the first electrode may comprise analkali-containing transition metal layer which is formed by sputteringfrom a target of one of the embodiments of the present invention. Forexample, the first electrode may be formed by sputtering from a targetthat comprises a sputtering layer comprising a first material. The firstmaterial may comprise an alkali containing transition metal sputteringlayer. The target may additionally comprise a support structurecomprising a second material. The sputtering layer of the target maydirectly contact the second material, and the second material isselected such that atoms which negatively impact the performance of thep-type absorber layer are not incorporated into the p-type semiconductorabsorber layer from the alkali containing transition metal layer of thefirst electrode.

A modular sputtering apparatus for making the solar cell, as illustratedin FIG. 10 (top view), may be used for depositing the layers of thesolar cell discussed above. The apparatus is equipped with an input, orload, module 21 a and a symmetrical output, or unload, module 21 b.Between the input and output modules are process modules 22 a, 22 b, 22c and 22 d. The number of process modules 22 may be varied to match therequirements of the device that is being produced. Each module has apumping device 23, such as vacuum pump, for example a high throughputturbomolecular pump, to provide the required vacuum and to handle theflow of process gases during the sputtering operation. Each module mayhave a number of pumps placed at other locations selected to provideoptimum pumping of process gases. The modules are connected together atslit valves 24, which contain very narrow low conductance isolationslots to prevent process gases from mixing between modules. These slotsmay be separately pumped if required to increase the isolation evenfurther. Other module connectors 24 may also be used. Alternatively, asingle large chamber may be internally segregated to effectively providethe module regions, if desired. U.S. Published Application No.2005/0109392 A1 (“Hollars”), filed on Oct. 25, 2004, discloses a vacuumsputtering apparatus having connected modules, and is incorporatedherein by reference in its entirety.

The web substrate 100 is moved throughout the machine by rollers 28, orother devices. Additional guide rollers may be used. Rollers shown inFIG. 10 are schematic and non-limiting examples. Some rollers may bebowed to spread the web, some may move to provide web steering, some mayprovide web tension feedback to servo controllers, and others may bemere idlers to run the web in desired positions. The input spool 31 aand optional output spool 31 b thus are actively driven and controlledby feedback signals to keep the web in constant tension throughout themachine. In addition, the input and output modules may each contain aweb splicing region or device 29 where the web 100 can be cut andspliced to a leader or trailer section to facilitate loading andunloading of the roll. In some embodiments, the web 100, instead ofbeing rolled up onto output spool 31 b, may be sliced into solar modulesby the web splicing device 29 in the output module 21 b. In theseembodiments, the output spool 31 b may be omitted. As a non-limitingexample, some of the devices/steps may be omitted or replaced by anyother suitable devices/steps. For example, bowed rollers and/or steeringrollers may be omitted in some embodiments.

Heater arrays 30 are placed in locations where necessary to provide webheating depending upon process requirements. These heaters 30 may be amatrix of high temperature quartz lamps laid out across the width of theweb. Infrared sensors provide a feedback signal to servo the lamp powerand provide uniform heating across the web. In one embodiment, as shownin FIG. 10, the heaters are placed on one side of the web 100, andsputtering targets 27 a-e are placed on the other side of the web 100.Sputtering targets 27 may be mounted on dual cylindrical rotarymagnetron(s), or planar magnetron(s) sputtering sources, or RFsputtering sources.

After being pre-cleaned, the web substrate 100 may first pass by heaterarray 30 f in module 21 a, which provides at least enough heat to removesurface adsorbed water. Subsequently, the web can pass over roller 32,which can be a special roller configured as a cylindrical rotarymagnetron. This allows the surface of electrically conducting (metallic)webs to be continuously cleaned by DC, AC, or RF sputtering as it passesaround the roller/magnetron. The sputtered web material is caught onshield 33, which is periodically changed. Preferably, anotherroller/magnetron may be added (not shown) to clean the back surface ofthe web 100. Direct sputter cleaning of a web 100 will cause the sameelectrical bias to be present on the web throughout the machine, which,depending on the particular process involved, might be undesirable inother sections of the machine. The biasing can be avoided by sputtercleaning with linear ion guns instead of magnetrons, or the cleaningcould be accomplished in a separate smaller machine prior to loadinginto this large roll coater. Also, a corona glow discharge treatmentcould be performed at this position without introducing an electricalbias.

Next, the web 100 passes into the process modules 22 a through valve 24.Following the direction of the imaginary arrows along the web 100, thefull stack of layers may be deposited in one continuous process. Thefirst electrode 202 may be sputtered in the process module 22 a over theweb 100, as illustrated in FIG. 10. Optionally, the process module 22 amay include more than one target, for example a first alkali-containingtransition metal target 27 a and a second alkali-containing transitionmetal target 27 b, arranged in such a way that each alkali-containingtransition metal target has a composition different from that of thetransition metal target. Furthermore, targets 27 a and 27 b may beselected from the targets of the various embodiments of the presentinvention, for example any of targets 1 a-d, or 2 a-c as illustrated inFIGS. 1-7.

The web 100 then passes into the next process module, 22 b, fordeposition of the at least one p-type semiconductor absorber layer 301.In a preferred embodiment shown in FIG. 8, the step of depositing the atleast one p-type semiconductor absorber layer 301 includes reactivelyalternating current (AC) magnetron sputtering the semiconductor absorberlayer from at least one pair of two conductive targets 27 c 1 and 27 c2, in a sputtering atmosphere that comprises argon gas and aselenium-containing gas. In some embodiment, the pair of two conductivetargets 27 c 1 and 27 c 2 comprise the same targets. For example, eachof the at least two conductive targets 27 c 1 and 27 c 2 comprisescopper, indium and gallium, or comprises copper, indium and aluminum.The selenium-containing gas may be hydrogen selenide or selenium vapor.In other embodiments, targets 27 c 1 and 27 c 2 may comprise differentmaterials from each other. The radiation heaters 30 maintain the web atthe required process temperature, for example, around 400-800° C., forexample around 500-700° C., which is preferable for the CIS based alloydeposition.

In some embodiments, at least one p-type semiconductor absorber layer301 may comprise graded CIS based material. In this embodiment, theprocess module 22 b further comprises at least two more pairs of targets(227, and 327), as illustrated in FIG. 4. The first Magnetron pair 127(27 c 1 and 27 c 2) are used to sputter a layer of copper indiumdiselenide while the next two pairs 227, 327 of magnetrons targets (27 c3, 27 c 4 and 27 c 5, 27 c 6) sputter deposit layers with increasingamounts of gallium (or aluminum), thus increasing and grading the bandgap. The total number of targets pairs may be varied, for example may be2-10 pairs, such as 3-5 pairs. This will grade the band gap from about 1eV at the bottom to about 1.3 eV near the top of the layer. Details ofdepositing the graded CIS material is described in the Hollars publishedapplication, which is incorporated herein by reference in its entirety.

Optionally, one or more process modules (not shown) may be added betweenthe process modules 21 a and 22 a to sputter a back side protectivelayer over the back side of the substrate 100 before the electrode 200is deposited on the front side of the substrate. U.S. application Ser.No. 12/379,428 (Attorney Docket No. 075122/0139) titled “ProtectiveLayer for large-scale production of thin-film solar cells” and filed onFeb. 20, 2009, which is hereby incorporated by reference, describes suchdeposition process. Further, one or more barrier layers 201 may besputtered over the front side of the substrate 100 in the processmodule(s) added between the process modules 21 a and 22 a. Similarly,one or more process modules (not shown) may be added between the processmodules 22 a and 22 b, to sputter one or more adhesion layers 203between the alkali-containing transition metal layer 202 and the CIGSlayer 301.

The web 100 may then pass into the process modules 22 c and 22 d, fordepositing the n-type semiconductor layer 302, and the transparent topelectrode 400, respectively. Any suitable type of sputtering sources maybe used, for example, rotating AC magnetrons, RF magnetrons, or planarmagnetrons. Extra magnetron stations (not shown), or extra processmodules (not shown) could be added for sputtering the optional one ormore Anti-Reflection (AR) layers.

Finally, the web 100 passes into output module 21 b, where it is eitherwound onto the take up spool 31 b, or sliced into solar cells usingcutting apparatus 29. While sputtering was described as the preferredmethod for depositing all layers onto the substrate, some layers may bedeposited by MBE, CVD, evaporation, plating, etc., while, preferably,the CIS based alloy is reactively sputtered.

It is to be understood that the present invention is not limited to theembodiment(s) and the example(s) described above and illustrated herein,but encompasses any and all variations falling within the scope of theappended claims. For example, as is apparent from the claims andspecification, not all method steps need be performed in the exact orderillustrated or claimed, but rather in any order that allows the properformation of the solar cells of the present invention.

1. A method of forming a sputtering target, comprising: forming asputtering layer comprising a first material which comprises analkali-containing transition metal directly on a second material of asupport structure, wherein the second material does not negativelyimpact a performance of a copper indium selenide (CIS) basedsemiconductor absorber layer of a solar cell.
 2. The method of claim 1,wherein: the support structure comprises a barrier layer comprising thesecond material formed on a backing structure of a third material thatis different from the second material; and the barrier layer is capableof blocking diffusion of atoms from the backing structure into thesputtering layer.
 3. The method of claim 2, wherein: the barrier layeris formed between the sputtering layer and the backing structure.
 4. Themethod of claim 3, further comprising forming the barrier layer bydepositing the second material onto the backing structure.
 5. The methodof claim 4, wherein the depositing is done by sputtering or thermalspraying.
 6. The method of claim 1, wherein the support structurecomprises a backing structure comprising the second material.
 7. Themethod of claim 1, wherein the first material comprises 0.5 to 10 wt %sodium, 0-50 wt % oxygen and balance molybdenum.
 8. The method of claim1, wherein the sputtering layer comprises sodium and oxygen containingmolybdenum.
 9. The method of claim 1, wherein the support structurefurther comprises a barrier layer formed between the sputtering layerand the backing structure.
 10. The method of claim 9, wherein thebarrier layer comprises at least one of Nb, Cr and Mo.